Over the recent years, the market has shown a trend of increasing adoption of TV applications to be included in PCs and set-top boxes, and this trend is not foreseen to stop. Today, devices exist that already include 2 TV receivers enabling to watch a TV program while recording another TV program. In the near future, TV receivers should at least be capable of simultaneously receiving four programs (e.g. in cable TV applications) and maybe even more.
With all those broadband TV receivers in a small form factor and on the same die, the issue of VCO pulling becomes increasingly important. With small(er) “form factor” is meant that the integration surface on silicon is getting smaller and smaller which makes interactions among sensitive blocks and EM sources (like VCOs) stronger. It has become a challenge to let multiple receivers operate at the same time without degrading the performance of the individual devices to a point where reception quality becomes unacceptable.
VCO pulling may already be observed today in systems with two TV receivers, in case a user desires to watch one TV channel and to record another TV channel. For example, a voltage-controlled oscillator (VCO1) in the first receiver is locked to incoming data of TV channel 1 at fVCO1 and a voltage-controlled oscillator (VCO2) in the second receiver is locked to incoming data of TV channel 2 at a slightly different frequency fVCO2. The two oscillators pull each other as a result of coupling through the substrate, the package, and the power supply (VCC/GND). As a consequence, the VCO1 output spectrum contains some sideband components at fVCO1 +/−abs(fVCO1-fVCO2) where abs(x) is the absolute value of x. Similarly, the VCO2 output spectrum contains some sideband components at fVCO2 +/−abs(fVCO1-fVCO2). These sidebands will also demodulate in the base-band some unwanted RF signal at frequency fRF +/−abs(fVCO1-fVCO2) leading to an increase of noise. Therefore some sensitivity loss is most likely to be observed and the picture quality of the watched and/or recorded program may be deteriorated till an unacceptable level.
VCO pulling may also be observed in a 3G/CDMA2000 full-duplex transceiver when transmitting and receiving VCOs are operating simultaneously at different frequencies.
VCO pulling may also be observed in wireless voice-over-IP applications over WLAN, where the user desires to use a Bluetooth headset. As WLAN and Bluetooth operate on the same frequency band, the frequencies of the Bluetooth (BT) and WLAN VCOs may be very close. VCO pulling may be observed when, for example, the Bluetooth device B is transmitting a burst to the Bluetooth headset, and the
WLAN device is receiving data at the same time.
The above described problems resulted in a need for reducing the coupling between the VCO's, in particular between inductors in these VCO's. In the prior art, the so-called integrated 8-shaped inductor (as part of a semiconductor device) is proposed as a solution. Such 8-shaped inductor exhibit a reduced strength of the radiated magnetic field (in the far field). The magnetic field generated by one of the loops is (in the far field) compensated by the magnetic field generated by the other loop.
A problem with the known 8-shaped inductor is that the residual far-field is still too large.